<!DOCTYPE html>
<html lang="zh-CN">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>15-5秒跳转页面</title>
</head>

<body>
  <button>点击跳转</button>
  <div></div>
  <script>
    var btn = document.querySelector('button');
    var div = document.querySelector('div');
    btn.addEventListener('click', function () {
      location.href = 'http://www.itcast.cn';
    })

    var timer = 5;
    
    setInterval(function () {
      if (timer == 0) {
        location.href = 'http://www.itcast.cn';
      } else {
        div.innerHTML = '您将在' + timer + '秒后跳转至首页'
        timer--;
      }
    }, 1000)
  </script>
</body>

</html>